1. Field of the Invention
The invention disclosed herein relates to method and apparatus for imaging a structure downhole and, in particular, to imaging using a fiber optic sensor.
2. Description of the Related Art
Hydrocarbons are generally recovered through boreholes penetrating reservoirs of the hydrocarbons. Various types of structures may be disposed in the boreholes for the recovery process. During well completion, one type of structure known as a casing or tubular is disposed in a borehole. The casing, similar to a pipe, is used to contain the hydrocarbons flowing to the surface of the earth for recovery.
Structures such as casings disposed in boreholes can be exposed to harsh environments. The harsh environments include high temperature, high pressure, and high stress. The high stress can result from the high temperature, the high pressure, high loads or high vibration. Because of the high cost of well completion, it is important to monitor the downhole environment and strains experienced by the downhole structures to prevent damage.
Monitoring the downhole environment and the downhole structures requires both a communication medium and sensors that are able to withstand a harsh environment. Optical fibers disposed in cables known as a fiber optic cable are used as the communication medium and as sensors in the borehole because the fiber optics have characteristics which make them suitable to withstand the harsh environment encountered in the borehole.
Because more than one fiber optic sensor may be used in one borehole, splices are required to couple each fiber optic sensor to an assigned optical fiber in a fiber optic communication cable. While fiber optics may be able to withstand the pressures and temperatures of the harsh downhole environment, fiber optics are delicate and require careful and precise installation especially when installing the splices. Unfortunately, installing the splices on the floor of a completion rig is not conducive to the care and precision required for a reliable and working installation.
Another problem can arise when a faulty sensor is first inserted into the borehole before a splice is made coupling the sensor to the fiber optic communication cable. In this case, the faulty sensor can be far down the borehole before the fault is detected resulting in wasted time to remove the sensor from the borehole for repair or replacement. This is especially true when the sensor is a fiber optic with embedded fiber Bragg gratings used for real time casing imaging (RTCI).
In RTCI, the fiber optic with the embedded fiber Bragg gratings is wrapped around a section of the casing in a spiral shape. The fiber optic is then epoxied in place and sometimes wrapped in tape. Thus, much time is required for a custom installation, and the rework time to repair or replace a faulty RTCI fiber optic sensor can be considerable.
The sensor is typically inserted into the borehole before the splice because the conventional splice housing is an “upside-down Y” having one uphole leg and two downhole legs. The uphole leg is connected to a fiber optic cable leading to the surface of the earth. The two downhole legs are generally connected to different sensors.
Therefore, what are needed are techniques for deploying a fiber optic communication cable and a fiber optic strain sensor in a borehole. Preferably, the techniques enable testing the strain sensor prior to the strain sensor being inserted into the borehole.